The invention relates to a method and a packing having a sealing element made of an elastomeric material for the pressure-balanced sealing off of a liquid or gaseous medium, especially in the clearance between machine parts which are movable relative to one another.
It is known to use shaft packing rings, such as for example those described in German Standard DIN 3760, for the sealing off of liquids. These shaft packing rings consist generally of a stiffening ring of metal which has an angular profile, a sealing element made of an elastomeric material being vulcanized onto the inwardly directed leg of said profile, with the interposition of a membrane. The sealing element has a sealing lip which usually is urged by a superimposed tension spring against the shaft.
The use of packings of this kind generally involves the disadvantage that they assure a satisfactory sealing action up to pressures not exceeding 10 kp/cm.sup.2, even when they are of the reinforced type. If the packing must withstand higher pressures, it is necessary either to arrange a plurality of such packings in series, also utilizing a hydraulic counterpressure system in some cases, or to use a different type of packing system such as a friction ring packing. Neither packing, however, will be satisfactory in all cases. Friction ring packings have the disadvantage of an extraordinarily high cost, and when shaft packing rings arrayed hydraulically in series are used, it is often impossible to achieve sufficient reliability of operation.
In the effort to improve the pressure stability of the single packing ring and thus combine the advantage of its ease of manufacture with a higher performance characteristic, difficulties are encountered due to the fact that the pressure resistance of these packing rings is limited by a number of technical circumstances.
One of the most important factors is probably that the applied hydraulic pressure results immediately in an increase of the force with which the sealing lip is urged against the surface of the moving part, which can produce uncontrolled overheating of the elastomeric material forming the sealing lip or in alterations of the angle of attitude of the sealing lip to the said surface.
Another difficulty can result from the fact that it is generally impossible to assume that the pressure of the applied medium is constant. For example, when a flange packing is used in a pump, each individual start-up of the machine will produce a gradual rise of the pressure up to the nominal pressure, and under certain circumstances it is also necessary to pass through critical rotatory speeds at which great shaft vibrations are produced. A statically set force of application of the sealing lip to the moving surface, based on a nominal value, thus precludes the use of such packings in these applications.